Investigating the turbulent structure of ionospheric plasma velocities on open and closed magnetic field lines

In this paper we review some recent work investigating the turbulent structure of the ionospheric plasma velocity in the polar and auroral regions. The studies make use of spatially distributed measurements of the ionospheric plasma velocity made with the Halley Super Dual Auroral Radar Network (SuperDARN) radar between 1996 and 2003. The spatial structure is analyzed using structure functions, which provide a simple method for deriving information about the scaling and intermittency of the fluctuations that can be used to differentiate between different turbulence models. Evidence for scale-free structure in regions of both open and closed magnetic field line topology is found, though with different power-law exponents. It is suggested that these different exponents arise as a result of different origins of the scale-free structure in the different regions, i.e., that seen on open magnetic field lines is a reflection of the spatial structure of the solar wind and that seen on closed magnetic field lines is dictated by the internal dynamics of the magnetosphere-ionosphere system. It is found that the limited range of velocities that can be measured by the Halley SuperDARN radar restricts the ability to calculate structure functions, though this can be corrected for by using conditioning (removing velocity fluctuations larger than 3 standard deviations from our calculations). The resultant structure functions suggest that Kraichnan-Iroshnikov versions of P and log-normal models of turbulence best describe the velocity structure seen in the nightside F-region ionosphere, poleward of the open-closed magnetic field line boundary (OCB), i.e., in regions magnetically connected to the turbulent solar wind.